Liquid crystal panels in liquid crystal display devices have a plurality of switching elements or thin-film transistors (hereinafter, TFTs) that are arranged in a matrix (rows and columns) in order to control the operation of respective pixels. Conventionally, silicon semiconductors such as amorphous silicon were generally used as semiconductor films for TFTs. However, the usage of oxide semiconductors having high electron mobility as semiconductor films has been proposed recently. Patent Documents 1 to 3 disclose liquid crystal display devices adopting TFTs using these types of oxide semiconductors as switching elements. The usage of oxide semiconductors having high electron mobility can provide improvements such as size reduction of the TFTs compared to conventional products and improvement in the aperture ratio of the liquid crystal panel.